TGeneratorPhaseSpace invariant mass

[lm17n000032]

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ROOT Version: 6.12/04
Platform: macOS Mojave 10.14.1
Compiler:
Hi ROOTers,

I tried to write a code to simulate invariant mass of a specific particle. The code is:

void massa_invariante() {
    TLorentzVector rho(0.0, 0.0, 0.0, 0.770);
    
    Double_t masses[2] = {0.139, 0.139} ;

    TGenPhaseSpace event;

    event.SetDecay(rho, 2, masses);
    
    TH1F mass_inv("Note","Distribuzione in massa invariante del processo #rho#rightarrow#pi^{+}#pi^{-}", 1000,0,2);
    
    for (Int_t n=0;n<1000000;n++) {
        

        Double_t weight = event.Generate();
        
        TLorentzVector *pmeno = event.GetDecay(0);
        TLorentzVector *ppiu = event.GetDecay(1);
     
        mass_inv.Fill((pmeno+ppiu).Mag() ,weight);
    }
    
    TCanvas *c1=new TCanvas();
   
    mass_inv.SetXTitle("M_{#pi^{+}#pi^{-}} GeV");
    mass_inv.SetYTitle("N");
    
    mass_inv.DrawCopy();
    
    c1->SaveAs("mass_inv.pdf");
}
   

the output generate a fake rho(770) invariant mass. Why this happened?

mass_inv.pdf (17.8 KB)

Someone could help me?
Thanks.

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[Wile_E_Coyote]

You should use: (*pmeno + *ppiu).Mag()

You set the total energy of “rho” at rest to 0.770 so why are you surprised that you get it back?

[lm17n000032]

The code works like before.
You are right, it is a doubt that I had previously. I do not know how to build the program, I have not been taught and I am making attempts. I would like to generate a defined number of pions + and pions - and build invariant mass of mother particle with them. I don’t find anything about it on the web . Can you give me some hint or reference? I do not have other ideas …